1. Field of the Invention
The invention relates to a process for the control of a vacuum evaporation process. Pursuant to the process, functional coatings are applied above all to tools and sheet substrates, such as mirrors and glass panes, through vapor deposition.
2. Discussion of Background Information
In vapor deposition, strict requirements are set with regard to the evenness of the coating thickness, the stability of the vapor deposition rate over long periods of time and the composition of the coating material. Especially in the vaporizing of substrates with large surfaces, these requirements can only be met if the parameters of the vapor deposition process are maintained at a constant level. This presupposes that the evaporation parameters, especially the rate of evaporation, the level of the vaporizer crucible, and the direction distribution of the vapor are maintained at a constant level or are adjusted.
The material to be vaporized is in a vaporizer. This can be a vaporizer crucible, for example, in which the material--the vaporizing material--is melted. These vaporizers are arranged horizontally. Sublimation vaporizers are also used in which the vaporizing material is lodged in a holding device and evaporated from the solid phase by supplying energy, such as electron beams or resistance heating. These vaporizers may be arranged in a horizontal, vertical or diagonal manner.
It is customary to measure the rate of evaporation in one or several locations near the substrate. There are a large number of processes for determining the rate of evaporation, such as the measuring of the thickness of coatings with the help of a quartz resonator or microbalance in the vicinity of the substrate (G. Kienel: Vakuumbeschichtung Band 3--Anlagenautomatisierung, VDI-Verlag, Dusseldorf, 1994, pp. 25-, pp. 35-, pp.40-). The rate of evaporation is determined based on these processes for measuring the thickness of coatings. The rate of evaporation and therefore the evaporation process is controlled via a control circuit with the obtained signal. The substantial disadvantage of these processes consists of the set-up of the measuring instruments. The sensors must be arranged inside the vapor room and thus are subjected to such high vapor and heat so that they must be replaced after only a relatively short period of time. Thus, a frequent interruption of the evaporation process is necessary and a long-term operation is practically impossible. For this reason, these processes are suitable for low evaporation rates only. In addition, only a small part of the vapor flow is measured, from which the entire vapor flow that reaches the substrate is inferred. This is especially disadvantageous in the case of large-surface substrates, since the sensors, in an effort not to cover the substrate, are positioned at a different distance and angle to the vaporizer than the substrate and since the vapor flow density to the sensor thus has a different value than the vapor flow density to the substrate. This results in errors.
In order to avoid these errors, there are other measuring methods for controlling the process that stand in relation to the vapor flow between vaporizer and substrate. The evaluation of emitted plasma beam (DD 239 810 A1), the determination of the absorption of light or laser beams in vapor (Gogol, C. A., Reagan, S. H., J. Vac. Sci. Technol. A1 (2), April/June 1983, pp. 252-256), or the evaluation of the ionization of the vapor with the help of electron bombardment (Hegner, F.: Anwendung der Elektronen-Emissionsspektroskopie fur das ratengeregelte Aufdampfen von Legierungen, Vak. Techn. 29, 2 (1980), pp. 45-49) are all known to that end. In these processes, the sensors are not located immediately in the vapor flow but are offset to the side of the vapor flow. These processes, too, are indirect measuring processes in which substantial disadvantage exists in that, aside from the vapor density, the measurable variables depend to a great extent upon additional factors, such as the type of the vaporizing material, the stimulation and ionization of the vapor, and the vapor velocity. The set-up of the measuring instruments thus must be calibrated anew for each vaporizing material, which is quite labor intensive. Another disadvantage is that the sensors must be arranged in the vapor room and therefore are subject to a gradual coating in turn. Thereby, interruptions of the process are likewise necessary so that the sensors can be replaced, which is disadvantageous for long-term operation.
It is widely known to determine the rate of evaporation as the temporal mean by weighing the vaporizing material by interrupting the evaporation process over longer time intervals. However, this process is not suitable for industrial applications since the rate of evaporation is determined only over large time intervals and thus is not available for a continuous control of the evaporation process, or the evaporation process would have to be interrupted constantly for weighing.
Another process is known for monitoring the level of the vaporizer crucibles by means of light or laser beams. In this process, a beam is reflected on the surface of the vaporizing material and the level of the vaporizer crucible is inferred from the beam path (DE 38 27 920 A1). This process has the disadvantage that the beam path may be disturbed by possible wave movements of the surface of the vaporizing material. In addition, this process has the disadvantage that the beam--in that it must be guided through areas with the highest degree of vapor density--is subjected to a dispersion and an absorption which results in a distortion of the results of the measuring process. For this reason, this process only is conditionally suitable for controlling an evaporation process so as to meet the stringent requirements for the stability of the rate of evaporation and the exact composition of the vapor-deposited coating.
A method for vapor depositing photosensitive material (EP 0 028514 A1) is known. In this process, the vaporizer crucible is weighed in order to determine the rate of evaporation of the photosensitive material. The energy that is to be applied to the vaporizer crucible is controlled in relation to the weight change and the temperature in the vaporizer vessel.